News Navigator: Why is a COVID-19 vaccine booster shot necessary?

The Mainichi Shimbun answers some common questions readers may have about a third dose of COVID-19 vaccines.

Question: Will we need to get a third dose after receiving two initial shots?

Answer: The third dose, or a booster shot, to further reinforce the vaccinated individual's immunity to the coronavirus has already been carried out in Israel. Countries including the United States and the United Kingdom are planning to start their own booster injections from September. Japan is also considering offering third shots.

Q: Why is a third dose necessary?

A: It's because we need to prolong the effect of the vaccine to prevent infections. Generally speaking, antibody levels acquired through vaccination and natural infection reportedly fall as time passes by. Amid the spread of coronavirus mutant variants including the highly transmissible delta one, infections among fully vaccinated people have been reported.

Q: Will a booster shot be effective in providing greater prevention?

A: The Israeli health institute, which is carrying out vaccinations there, has released a report showing an interim result that, compared to those who received two vaccine shots, the number of those who had developed post-vaccination COVID-19 was 86% lower among booster shot recipients. However, there still remain unanswered questions regarding the booster shots, such as how long the vaccine stays effective after the third dose. Coronavirus infections cannot be completely prevented just by vaccines, and preventive measures including wearing masks should be continued even after a third shot.

Q: A booster shot sounds like a good idea then, doesn't it?

A: Well, there are still many people in the world, mainly in developing countries, who have been unable to get their first vaccine shot. The World Health Organization is urging first and foremost for the supply of vaccines to countries where distribution has been slow citing the lack of data requiring the third shot. Tetsuo Nakayama, specially appointed professor at Kitasato University in Tokyo who specializes in clinical virology, says, "If infection outbreaks do not subside in areas where vaccinations are not steadily progressing, it will be easier for new variants to emerge, and we won't be able to contain the coronavirus." He added, "In Japan, too, priority should be given to administering two shots to those who wish to be vaccinated."

(Japanese original by Ayumu Iwasaki, Science and Environment News Department)